Final answer:
Insufficient incubation time is the error that would result in positive results for all samples in an ELISA assay, including negative controls.
Step-by-step explanation:
The error that would result in positive results for all samples in an ELISA assay (including negative controls) is 4) Insufficient incubation time. Insufficient incubation time would lead to incomplete binding of the primary and secondary antibodies to the target antigen, resulting in false positives for all samples. For example, if the incubation time is too short, the secondary antibody may not have enough time to bind to the primary antibody, resulting in false positive signals.